Living body communication in which communication data are transmitted and received through living bodies such as human bodies has been drawing attention. The living body communication is expected to be applied to medical and health care services, short-range wireless communication practices, in-vehicle wireless communication operations, amusement activities, and the like. The living body communication can significantly reduce power consumption as compared to some conventional communication techniques.
In the living body communication, each communication apparatus configured to use a living body as part of a transmission path is provided with two electrodes. One is a signal electrode connected to a signal line, and the other is a reference potential electrode connected to ground potential of the communication apparatus having reference potential. The signal electrodes of the respective communication apparatuses are electrically connected together mainly through the living body, while the reference potential electrodes of the respective communication apparatuses are electrically connected together mainly through the space or the earth. Thereby, each communication apparatus transmits the potential difference between the signal electrode and the reference potential electrode to the counterpart communication apparatus.
Each communication apparatus to perform the living body communication uses the electrostatic coupling between the electrodes and the surface of the living body. Accordingly, the capacitance value of the electrodes decreases as the distance of the communication apparatus from the surface of the living body becomes larger. A potential effective solution to the decrease in the capacitance value of the electrode is the enlargement of the area of each electrode, or adoption of a variable capacitance element. However, a problem with the enlargement of the electrode area is that the size of the electrode area is restricted by an apparatus that embeds the electrodes. Meanwhile, the adoption of the variable capacitance element poses a problem that the communication is likely to take place even if there is no living body around the apparatus unless a distance between the apparatus and such a living body is recognizable.
For this reason, a living body detection sensor capable of recognizing the distance between an apparatus and a living body is essential for the apparatus to perform the living body communication. Furthermore, the living body detection sensor is applicable as a metal detection sensor to detect the location and shape of a piece of metal which is implanted in an object.